Due to the rapid development of liquid-crystal display (LCD), the light-emitting diode (LED) backlighting is gradually becoming more common instead of the cold-cathode fluorescent lamps (CCFL) backlighting.
With reference to FIG. 1 for a boost LED backlight driver having a dimming and over-voltage protection device, comprises: a boost circuit 10, a driven device 20 (e.g. LED), a transistor Q1, a feedback resistor RFB, voltage dividing circuit 30, a modulation comparator 70, an over-voltage comparator 80 and a modulation signal source 50.
When a transistor Q3 is turned on, an input voltage Vin is applied across a inductor L1 so as to linearly increase the current of the inductor L1 to store energy therein. If the transistor Q3 achieves a desired on time, the transistor Q3 is turned off immediately such that the stored energy is outputted to an output terminal of the boost circuit 10 via a diode D1 and charges a capacitor C3. The input voltage Vin will increase to a predetermined level of the output terminal of the boost circuit 10 by repeating the aforementioned operations. These operations are referred to as the principle of boost.
Wherein, the output terminal of the boost circuit 10 is electrically coupled in series to the LED, the transistor Q1 and the feedback resistor RFB sequentially. A feedback voltage of the feedback resistor RFB is outputted to a FB pin and compared with an error amplifier (not shown) to generate a suitable duty cycle for outputting to the transistor Q3, and a predetermined current is achieved.
Wherein, an over-voltage protection method is the output voltage of the boost circuit 10 for outputting to an OVP pin via voltage dividing circuit 30 and comparing with a reference voltage 2.5V of the over-voltage comparator. If the output voltage exceeds 2.5V, the driving circuit is turned off and latched.
Besides, a dimming method is the modulation signal of the modulation signal source 50 for outputting to a DIM pin and comparing with a reference voltage of the modulation comparator 70, wherein the modulation signal is a pulse width modulation (PWM) signal. If the voltage of the modulation signal falls below 0.2V, the modulation comparator 70 outputs a signal to DIMOUT pin such that the transistor Q1 is turned off for cutting off the current of the LED. If the voltage of the modulation signal exceeds 0.3V, the modulation comparator 70 outputs a signal to DIMOUT pin such that the transistor Q1 is turned on for generating the current of the LED. Therefore, the driving circuit is enabled to modulate the current of the LED according to the duty cycle of the modulation signal.